How do I avoid mesh collider overlapping? (see image)

Hello, I´ve searched for days to solve this problem, but I can't find the right solution. This is the situation: I have a dice prefab with a mesh collider attached. The mesh collider component has a different mesh applied with lower poly count and the option convex is checked.

The rigidbody component options are:

• mass=1

• drag∠ular drag=0

• use gravity=true

• is kinematic=false

• interpolate=none collision

• detection=discrete

This is the code where I apply force on rolling the dice:

 // cycle through dice instances
         foreach(GameObject dice in diceInstanceList){
             // check if current dice is in dice stack, if not roll it
             if(!stackHandler.Contains(dice)){
                 // reset position
                 dice.transform.position = diceSpawn.position;
                 // reset rotation
                 dice.transform.Rotate(rndRotation());
                 // setup physics
                 dice.rigidbody.isKinematic = false;
                 dice.rigidbody.useGravity = true;
                 dice.rigidbody.mass = GamePlaySettings.diceMass;
                 dice.rigidbody.drag = GamePlaySettings.diceDrag;
                 dice.rigidbody.angularDrag = GamePlaySettings.diceAngularDrag;
                 dice.rigidbody.velocity = Vector3.zero;
                 // show dice
                 dice.renderer.enabled = true;
                 // push dice towards the board
                 dice.rigidbody.AddForce(transform.forward * GamePlaySettings.DiceForce);
             } // - is dice in stack?
         } // - foreach dice

Everything works fine but there are some overlapping issues. I roll (rigidbody.AddForce) 6 dice at the same time and the dice are overlapping each other. Look at the picture:

How do I avoid these issues? What am I doing wrong?

Thanks for your help and time! Best, Arne